Justices returned early from their summer break for the first time in nearly three decades to hear arguments in the complex case, the results of which will guide next year's campaigns and those for years to come.
The Bush administration's Supreme Court lawyer was among those defending the tough legislation Congress passed last year.
Solicitor General Theodore Olson said the law was a response to a perception of corrupt politics — "the breakfasts, the lunches, the receptions, the dinners ... the relentless pursuit of big contributions."
Kenneth Starr, the former independent counsel who investigated President Clinton, argued for those challenging the constitutionality of the new campaign spending limits.
The law "intrudes deeply into the political life of the nation" and "in a word, goes too far," said Starr.
Groups as varied as the American Civil Liberties Union and the National Rifle Association are opposed to the 2002 law, which bans huge, unlimited donations to political parties known as "soft money," and tightens controls on political advertising in the weeks before an election.
Starr, the first of eight attorneys who would argue before the justices, said the law hamstrings local political parties, which work with national parties, and hurts grass-roots political efforts.
Justice Antonin Scalia seemed skeptical of the law, which he said was passed by incumbent lawmakers worried about protecting themselves. "There will be abuses under this law," he predicted.
Olson said Congress may not have fixed every potential abuse, but spent six years coming up with the plan.
The case attracted unusual attention. Television cameras lined the approach to the court Monday morning and a line of seat-seekers stretched down the plaza of the court building toward the street. Sens. John McCain, R-Ariz., and Russell Feingold, D-Wis., principal authors of the law under challenge, took seats side-by-side in the court.
The high court's decision is expected by the end of the year. In the meantime, the 2004 presidential election campaigns will continue operating under the new law.
"The 2004 campaign is not waiting for the starting gun. People are already halfway down the track," said John Podesta, a Democratic strategist who was chief of staff to President Clinton.
If the court strikes down the entire law, which is unlikely, or some parts, Congress can start working on another version to control how money is raised and spent in future elections.
The Supreme Court has put the case on a fast track, scheduling an off-term session before its October term for the first time since 1974, when justices ordered then-President Nixon to surrender Watergate-scandal tape recordings.
The court was to hear four hours of arguments, taking the rare step of allowing release of audiotapes at the end of the session.
The most closely watched court members were Chief Justice William H. Rehnquist and Justice Sandra Day O'Connor, two conservatives who are considered possible swing votes.
Members of the court's more liberal wing — Justices John Paul Stevens, David H. Souter, Ruth Bader Ginsburg and Stephen Breyer — were thought to have the least objections to the new law.
Justices most likely to oppose it as unconstitutional infringement of free speech were Scalia, Anthony M. Kennedy and Clarence Thomas.
The case is McConnell v. FEC, 02-1674.
CBSNews.com Legal Analyst Andrew Cohen writes: Whatever the Justices ultimately decide their ruling is likely to be as complicated and convoluted as the law is and as the oral arguments were. I think some Justices will agree on certain specific issues but disagree on others and that may make the decision difficult to interpret and apply.
There is no tidy, clean, simple way to address and resolve the many issues raised by the campaign finance legislation so I think we are going to get a decision from the Justices that includes shifting majorities from topic to topic. The hope, of course, is that whatever the Court decides, the Justices give enough guidance so that everyone knows what the rules are going forward.
Whatever the Justices decide I don't think this issue goes away after this ruling comes down. I think that Congress likely will react to whatever the Court decrees and that means we might see some of these same issues back before the Justices in a year or two, Cohen concludes.